In railway cab signaling and speed control systems, various vehicle-carried apparatus and circuits are employed for being selectively responsive to one of a plurality of speed command input signals. The speed command signals are picked up from the wayside by inductive coils located ahead of the front axle of the leading vehicle. Each of the signals are coded to indicate the authorized speed that the train or transit vehicle is permitted to travel in any given section or block of trackway as it proceeds along the route of travel. The picked-up signals, which are made up of a carrier frequency signal and a selected one of a plurality of code frequencies or rates, after amplification and demodulation, are applied to a code-following master relay. This relay, in turn, drives the code filters which are selectively responsive to the particular code rate. The code responsive filters are connected to decoding relays to provide a given cab signal indication to the trainman or operator in the cab of the locomotive or lead vehicle. It will be appreciated that in such cab signal speed control systems, it is an authorative requirement for the trainman or operator to take appropriate action within a given period of time after the reception of a more restrictive speed command. Thus, when a more restrictive or lower speed command signal is received on board the vehicle, the operator must immediately acknowledge a warning signal and begin decelerating the moving train to the newly prescribed speed or the emergency brakes will be automatically applied after an elapsed period of time to bring the train to a complete stop. In practice, the acknowledgment of the warning signal and the deceleration of the moving train or transit vehicle must take place as soon as possible after the reception of the more restrictive speed command in order to prevent the train or vehicle from going too fast and too far beyond a safe braking and stopping point. Additionally, in order to provide the highest degree of safety to individuals and to afford the greatest protection against damage to the equipment, it is essential to ensure that under no circumstance will a critical component or circuit failure be capable of producing an unsafe condition. In the past, electronic acknowledging circuits were relatively expensive to initially purchase due to the excessive number of components and were costly to subsequently maintain due to the complexity of the circuitry. Thus, it is highly advantageous to develop a failsafe electronic acknowledging circuit which is simple in design and inexpensive to purchase and maintain.